Dr. Hovind (R4): It is very difficult or impossible to prove that a given sample has not been contaminated. Parent or daughter products could have leached in or out of the sample.
In the case of carbon-14 dating, the daughter product is ordinary nitrogen and plays no role in the dating process. We are only interested in tallying the original C-14 still present in the sample, the surviving "parent" isotope. The C-14 that is incorporated in the carbon structure of cellulose and the other structural materials of living plants and animals is not going to do much migrating after burial. If structural carbon migrated easily there soon wouldn't be any cellulose, lignin, chitin (or other structural carbon compounds) left in the soil! A piece of wood, for example, would soon turn into a formless cloud of graphite or soot in the soil, with perhaps a little ash marking the original shape! Clearly, that is not something which normally happens. Residues or solutions which do migrate can usually be washed out of the structural matrix of the sample with various chemicals.
To put it another way, we might imagine a piece of buried wood as being something like a sponge. Any carbon-containing liquid originally possessed by that sponge might well seep away over time, to be replaced by something else. However, unless the sponge itself disintegrates, the carbon which holds its fibers together must stay put. Thus, by choosing a sample that is structurally intact, one may rule out any significant loss of C-14. If the liquid impurities in our sponge can be washed and squeezed out, or estimated in some way, then we may be able to date the sponge (structural component of our sample) itself and get a good date even if non-structural carbon-14 had been lost in a manner that would upset the isotope ratio.
A sample, of course, can be contaminated if organic material rich in fresh atmospheric C-14 soaks or diffuses into it. Such contamination may occur in the ground or during the processing of the sample in the laboratory. However, such contamination will make the sample appear younger than its true age. Consequently, with regards to carbon-14 dating, creationists are barking up the wrong tree on the contamination issue!
Laboratories, of course, do have techniques for identifying and correcting contamination. There are various methods of cleaning the material, and the activity of each rinse can be measured. Lab contamination and technique can be checked by running blanks. A careful choice of samples will often minimize contamination. Dating various portions of a sample is another kind of check that may be performed.
Often there are cross-checks. Samples from top to bottom of a peat bog gave reasonable time intervals (Science, vol.200, p.11). The calibrated C-14 method confirmed Egyptian records, and most of the Aegean dates which were crossdated with Egyptian dates were confirmed (American Scientist, MayJune 1982). The marvelous agreement with tree-ring data, after correction for variations in the earth's magnetic field, has already been mentioned.
Carbon-14 dating thus presents a deadly challenge to young-earth creationists. If an old date is reasonably accurate, they're out of business; if an old date is bad due to contamination, then they are still out of business because the true date is most likely older still. It hardly seems fair, but that's the way it is. With that in mind, let's look at a few carbon-14 dates.
Egyptian barley samples have been found which date to 17,00018,300 years old (Science, April 7, 1978). On page 1346 the author explains some of the professional care which stands behind his use of the carbon-14 method.
A wooden walkway buried in a peat bog in England has been dated to about 4000 BC by the carbon-14 method (Scientific American, August 1990, p.30). Odd, that Noah's flood neither destroyed it nor deposited thick sediments on top of it! Jennifer Hillam of the University of Sheffield and Mike Baillie of Queen's University of Belfast and their colleagues were able to date the walkway by a second method, i.e., treering dating. They found out that the walkway, known as the Sweet Track, was built from trees felled in the winter of 38073806 BC. Pretty close agreement, huh?
Stonehenge, as dated by carbon-14, was built over a period from 1900 BC to 1500 BC -- long before the Druids came to England. Astronomer Gerald Hawkins found, after careful computer calculations, that the arrangement of the stones at Stonehenge are aligned with key positions of the sun and moon as they were almost 4000 years ago. (Weber, 1982, p.29). Thus, we have another remarkable confirmation of the C-14 method.
When did the volcano that destroyed Thera (and probably the Minoan culture as well) explode? Radiocarbon dating of seeds and wood buried in the ash, done by scientists at the University of Pennsylvania, pointed to no later than 1600 BC. Being that this was one of the biggest volcanic eruptions in recorded history, it almost certainly caused worldwide cooling which would, in turn, affect tree growth. Sure enough, the growth rings among oaks buried in Ireland's bogs show the effect of unusual cooling from 16281618 BC. Nor was that just an effect of local weather conditions. The bristlecone pines in the White Mountains of California show the same thing. A third estimate came from studies in Greenland. "In 1987 Danish geologists examining signs of volcanic acidity in the Greenland ice sheet concluded that the Thera volcano erupted in 1645 B.C., give or take 20 years." (Biblical Archaeology Review, Jan/Feb 1991, p.48). Thus, we have a remarkable agreement between three different methods, all within two or three percentage points of each other!
Trees buried by the last advance of glacial ice at Two Creeks, Wisconsin were dated at 11,850 years. (Strahler, 1987, p.251). Between those trees, which are buried in Valders red till, and an earlier, deeper layer of till, the Woodfordian gray till, lay the remains of a forest bed! What is a forest, including developed soil and rooted stumps, doing between two advances of ice? That could be an interesting question for someone who believes in only one "ice age." In 1878 Baron Gerard de Geer, a Swedish geologist, made a careful study of the annual varves left in European glacial lakes. By careful counting and crosschecking he was able to determine that the oldest glacial lakes, which would have formed at the start of the retreat of the ice, were 12,000 years old. Thus, we have a rough check between varves in glacial lakes and radiocarbon dating.
Richard Foster Flint, a professor of geology at Yale University and an expert on the Pleistocene epoch, was among the first to apply radiocarbon dating to glacial events. Collecting wood, bones and other organic material that had been covered over by the Laurentide Ice Sheet as it plowed across eastern and central North America, Flint collaborated with geophysicist Myer Rubin to demonstrate in 1955 that in most places the ice sheet achieved its greatest advance about 18,000 years ago, began to withdraw shortly thereafter and then hastened its retreat about 10,000 years ago.
Ancient cave art, at cueva de los caballos, near Castellon, Spain has been dated at about 6000 BC (The Times Atlas of World History ).
On the wall of Gargas Cave in the French Pyrenees are the outlined hands of Ice Age artists which date to at least 12,000 years. Magnificent prehistoric cave art, comparable to that of the world-famous caves of Altamira, Spain and Lascaux, France, was recently discovered in southern France, in the Ardeche River canyon area (Los Angeles Times ; Pasadena Star-News January 19, 1995). Its 300 paintings of such animals as bison, reindeer, rhinoceros, woolly rhinoceros, a panther, an owl, a hyena, bears, lions, horses, wild oxen, mammoths, wild goats and other animals is estimated to be between 19,000-22,000 years old. Sorry, no dinosaur drawings were reported! Such cave art, in Europe, was at its height around 20,000 years ago. Some examples probably go back 30,000 years!